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Thread: Mixng tank and tankless systems-Any reason not to?

  1. #1
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    Default Mixng tank and tankless systems-Any reason not to?

    Hi folks,


    Just found this site in the process of doing research on tankless systems. I looked through many posts and did not see one directly related to my situation.

    We are considering a shower remodeling. We want to put is a shower with the side body sprayers and large rain type shower head. The design we are considering can use as much as 7 gallons a minute. We have a 40 gallon propane water heater. With a 40 gallon tank and 7 gallons a minute for a shower, we can see there may be a problem with running out of hot water during a long shower after a long hard day's work.

    We are wondering what about a hybrid system with both the tank and tankless systems working together.

    What if the flow system has the tankless system after the tank system?

    Would it work as follows: the tankless system would not turn on until the water from the tank system started to fall below a certain value we set. We are thinking the tankless system would not be used a lot, but when it was needed it would come to the rescue.

    What do you folks with experience with such matters think?

    Thanks in advance for any help or comments.

  2. #2
    Moderator & Master Plumber hj's Avatar
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    Depending on the settings, either the first heater or the one set the hottest will do the majority of the heating.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    If you were to do this, you may find it worked better to put the tankless before the WH. The tank would give you the big buffer, and since it would be getting hot in instead of cold, it could extend the hot water much longer. The tank would prevent the low flow issues you have with a tankless, allow easier recirculation if you wanted it, and some tankless systems just don't like hot water coming into them. Plus, many tankless systems have a restrictor in them to limit flow. You'd want to be careful about that, as it would give you problems with the high flow shower.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Senior Member zl700's Avatar
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    Putting the tankless before the tank and the tank might as well not be there since the tankkless will regulate flow to heat water to proper temp thus, overall flow will be regulated and the tank just becomes a heated mass. If you circulated back from the tank to the inlet of the tankless, the water would be tempered, thus flow would be increased. Alot of hassles and added equipment besides the overall efficiency drops defeating 1/2 the premise of having a tankless.

    7 GPM isn't that much and perhaps a tankless alone could do the job with a proper model, or install two tankless.

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    Quote Originally Posted by hj View Post
    Depending on the settings, either the first heater or the one set the hottest will do the majority of the heating.
    Thanks for the feedback folks.

    It is my intension to have the tank heater set high enough that the tankless heater after it in the flow system will not turn on until there is a certain drop in the temperature from the tank heater. I am assuming since the tankless will be recieving "preheated" water that would require little additional heating and have little resaon to limit flow until the tank water temperature drops 30-40 degrees.

    I am thinking the issue would be getting the settings balance so the tankless only comes on when water heating demand is excessive.

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    DIY Senior Member zl700's Avatar
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    Tank before tankless is the best way. However tankless have a minimum flow rate, so when water enters it they typically fire at minimum , raising water temp higher even if you didn't want it. The tank set lower than set point of the tankless will help.

    Again the tankless would be a waste of money sitting there inline as a restriction. Better off with 2 tankless or 2 tanks, not 1 each.

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    Quote Originally Posted by zl700 View Post
    Tank before tankless is the best way. However tankless have a minimum flow rate, so when water enters it they typically fire at minimum , raising water temp higher even if you didn't want it. The tank set lower than set point of the tankless will help.

    Again the tankless would be a waste of money sitting there inline as a restriction. Better off with 2 tankless or 2 tanks, not 1 each.
    Thanks for the info. I just got through doscussing the situation with my LP company. They set the tank to about 60 degrees and let the tankless do the heavy work.

  8. #8

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    Here is a reason not to…It’s silly. If you need 7GPM total flow assuming you are using a tankless set at 120*F you are mixing in cold water at the fixture anyway. Figuring an 80/20 rule you need about 5.6GPM of hot water. ONE Rinnai R94LSi will give you that in unlimited supply! Now you will be at the capacity of the unit meaning that if you want to run other things at the same time as that fixture, you’ll see a reduction in flow. But really, how realistic is that? If you must run other things, consider dropping down to two R75LSi units and then you have a heck of a system that will give you upwards of 10GPM+ of endless hot water!

    May times people oversize tankless systems due to unrealistic system demands? You know how you use your house. Give it a little thought and you’ll be very happy.

    PS. Make sure your well/water supply can deliver what you want! A ¾” piping system can’t give you much more than about 8GPM total anyway.

    One more thing DO NOT SET YOUR TANK BELOW 120*!!!!!!! EVER! Legionnaires is a very real hazzard!!!!!!
    Last edited by Scott D. Plumber; 03-09-2010 at 08:05 AM. Reason: Needed to add danger post!

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    Quote Originally Posted by Scott D. Plumber View Post
    Here is a reason not to…It’s silly. If you need 7GPM total flow assuming you are using a tankless set at 120*F you are mixing in cold water at the fixture anyway. Figuring an 80/20 rule you need about 5.6GPM of hot water. ONE Rinnai R94LSi will give you that in unlimited supply! Now you will be at the capacity of the unit meaning that if you want to run other things at the same time as that fixture, you’ll see a reduction in flow. But really, how realistic is that? If you must run other things, consider dropping down to two R75LSi units and then you have a heck of a system that will give you upwards of 10GPM+ of endless hot water!

    May times people oversize tankless systems due to unrealistic system demands? You know how you use your house. Give it a little thought and you’ll be very happy.

    PS. Make sure your well/water supply can deliver what you want! A ¾” piping system can’t give you much more than about 8GPM total anyway.

    One more thing DO NOT SET YOUR TANK BELOW 120*!!!!!!! EVER! Legionnaires is a very real hazzard!!!!!!
    Thank you. Based on your comments and additional information I will be going with a tankless system only. We are going with a Noritz NR98-SV-LP (N0751M-LP) Tankless Water Heater. The 30% Federal Tax Credit is an added bonus.

    I do have a unique setup in that I have a underground holding tank for my water that is supplied from a shared well. The pump in the tank and one inch supply pipe can easliy provide in excess of 8GPM. Your comment on Legionnaires was the final nail in the tank coffin.

    I want to thank everyone here for providing me with all the help.

  10. #10

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    Mogo,

    You will very happy assuming everything is installed correctly. THe Noritz is top drawer. Don't forget the drain pan "The Wall Saver" if it installed in an area where a leak could cause damage. (Requiured by code and the instalation instructions.) This is often overlooked on tankless. and make sure you have the plumbing pipe kit serving it too for easier service later should it be needed.

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